Constant moment weigh scale with floating flexure beam

ABSTRACT

A weigh scale apparatus in which forces caused by the applied load are transformed into moments of opposite sense and applied to opposite ends of a substantially rigid beam. Deflection of the beam is measured by centrally disposed strain gage resistors to produce an electrical signal quantity proportional to the applied load. The conversion of applied load to moments of opposite sense is accomplished by a pair of spaced parallel pivot members which are suspended by straps between the load receiving pan and the base such that the pivots tend to rotate in opposite directions. The beam is mechanically connected between the pivots such that it is subjected to the bending moment over substantially its entire length. A split scale device is provided by a composite pan, one portion of which is connected to the pivots directly and another portion of which is connected to the pivots through long lever arms to provide moment multiplication. A vibrator device for dithering the pivots is provided. .Iadd.

This application and Serial No. 06/052077 are divisional reissueapplications of Serial No. 05/537,090, filed Dec. 30, 1974, which issuedas U.S. Pat. 3,938,603..Iaddend.

INTRODUCTION

This invention relates to weight scale apparatus and particularly to ascale which employes a beam-type flexure element and means to detectbending of the beam as a measure of applied load.

BACKGROUND OF THE INVENTION

Weigh scale apparatus, other than the laboratory balance type, typicallycomprises a spring element such as a coil or wrapped spring or a torsionbar which is deflected in proportion to applied loads. The traditionalmeans for sensing spring deflection are such that it is difficult andexpensive to attain a high degree of accuracy. Thus, an inexpensivedevice is typically not precise in its measurements. Moreover, the priorart scales are also typically quite sensitive to the point on the pan orload receiving surface at which the load is applied; i.e., anon-centered load on the pan tends to produce an inaccurate reading.

The present invention overcomes the disadvantages of the prior art byproviding a scale which can be manufactured so as to produce anextremely high degree of accuracy with relatively little expense andwhich, in a preferred form, is relatively insensitive to non-centeredload applications.

BRIEF SUMMARY OF THE INVENTION

As stated above, the present invention provides a weigh scale apparatushaving an entirely novel sensing principle which, because of thesimplicity of the implementation of the principle, is capable ofproducing high accuracy at relatively little cost. Moreover, theinvention is susceptible of implementation in various forms for a widevariety of applications ranging from food weighing scales forsupermarkets to livestock elevators, grain elevators, highway vehiclescales, to extremely sensitive devices such as those used in thelaboratory.

In general, the principle of the present invention involves thetransformation of an applied load into moments which in turn are appliedto opposite ends of a beam of high modulus of elasticity material so asto cause elastic deflection of the beam. Means such as strain gages areprovided for the measurement of the deflection stress in the beam as anindication of the applied load.

As hereinafter set forth in greater detail, the invention may beembodied in a weight scale apparatus having a base and a load receivingmember such as a pan, a pair of pivot members spaced apart and disposedmechanically between the pan and the base so as to experience a turningmoment when a load is applied to the pan. The term "pivot" as usedherein shall be understood to refer to a mechanical device which is somounted in the scale as to be subject to the turning moment when theload is applied, the actual rotation or angular deflection of such adevice being so minimal as to be substantially imperceptible to thehuman eye and thus requiring no elaborate or commonly conceived bearingsto allow for extensive angular rotation. Connected to and hence"floating" between the two pivot members is a beam of high modulus ofelasticity material such as steel or aluminum. By virtue of themechanical connection of the beam to the pivot members the turningmoments of the pivot members are applied to the opposite ends of thebeam thus to cause a bending deflection of the beam. Means are providedat the center of the beam for measuring the tension and compressionforces in the beam due to bending and for providing an output signalwhich is indicative of applied loads. The beam is so machined as todefine at the center thereof a flexure area of reduced cross sectionwhich tends to concentrate the bending stress in this area. Moreover,means such as strain gage resistors are bonded into intimate contactwith the flexure area so as to produce an electrical signal which, uponsuitable amplification, indicates the applied load as a function of thebending stress in the beam.

Various specific features of the invention are also described hereinincluding, as examples, a force multiplying lever arm arrangement and asplit pan; i.e., a pan having one area of a first scale sensitivity andanother area for a second scale sensitivity thus to be capable ofweighing objects in vastly dissimilar weight ranges to substantially thesame degree of accuracy; another example involves the use of a vibratoror similar device in the scale to dither the mechanical components ofthe pivot assembly thus to produce a "live" apparatus having nonoticeable friction or hysteresis characteristics. Various otherfeatures and advantages of the invention will become apparent from areading of the following specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a weigh scale apparatus embodying theinvention and comprising a digital readout or display device;

FIG. 2 is a plan view of a representative portion of the weigh scaleapparatus of FIG. 1 with the cover or pan removed so as to show theinterior details thereof;

FIG. 3 is a perspective view of representive interior details in theweigh scale apparatus of FIG. 1;

FIG. 4 is a side view partly in section of the interior details of theweigh scale apparatus of FIG. 1;

FIG. 5 is schematic circuit diagram of a bridge circuit for the bendingstress monitoring system applicable to the beam of the apparatus ofFIGS. 1 through 4;

FIG. 5a is a perspective view of the central portion of the beamillustrating the disposition of strain gage resistors on a flexure areathereof;

FIG. 6 is a schematic mechanical drawing of a strap-type pivot and beamsupport system of the weigh scale apparatus of FIGS. 1 through 4;

FIG. 7 is a perspective drawing of a split scale device;

FIG. 8 is a perspective drawing of representative interior details ofthe split scale device; and

FIG. 9 is a side view in section of a representative portion of thesplit scale device.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Looking first to FIG. 1 an illustrative weigh scale apparatus 10designed for general purpose usage is shown to comprise a flat pan 12approximately eleven inches by seventeen inches disposed on arectangular aluminum base 14 for receiving and weighing objects 16.Weigh scale apparatus 10 responds to objects 16 to produce an electricalsignal which is conducted through multiconductor cable 18 to a digitaltype display unit 20. The unit 20 is responsive to an electrical signalvoltage of varying amplitude to provide a temporary visual display of anumber which represents the weight of the object 16 in a selectedmeasurement system. Brightness and other operating characteristics ofthe unit 20 may be controlled as desired; the device 20 is acommercially available device and, while it is representative of one ofa wide variety of display or other readout devices which can be employedwith the invention, it actually forms no part of the invention and hencewill not be described in detail herein.

Looking now to FIGS. 2 through 4 of the interior details of the weighscale apparatus 10 will be described in greater detail. The pan 12 isessentially a rectangular body of substantially rigid material such asaluminum or steel having a depending peripheral lip or flange 13 as bestshown in FIG. 3 and having discretely located holes which receivefasteners so as to permit the pan to be intimately and closelymechanically secured to parallel rails 22a and 22b which are disposedwithin the confines of the base 14. The base itself comprises a thinfloor bounded by outer longitudinal rails 14a and 14b and side rails 14call secured together by machine screws, welds and combinations thereof.

Longitudinal rails 22a and 22b provide flat horizontal surfaces whichreceive the interior surface of the pan 12 thereon and hold the pan outof physical contact with any portion of the rectangular base 14.Longitudinal rails 22a and 22b are interconnected by transverselyextending members 24 only the lefthand member of which is shown in FIGS.2 and 3, it being understood that another such rail is reverselysimilarly arranged about the centerline of the assembly defined by thetransversely extending stiffener strap 25. The upper surface of railmember 24 is in the same plane as the upper surfaces of rails 22a and22b and hence helps support the flat interior surface of the pan 12.

Mechanically suspended between the pan 12 and the base 14 and out ofdirect physical contact therewith is a pair of reversely similar pivotmembers 26 made of heavy machined material such as aluminum, one suchpivot member at each longitudinal end of the assembly shown in FIGS. 2through 4. The suspension of pivot 26 is accomplished by steel strapassemblies 28 and 32 such that loads applied to the pan 12, such as byplacing an object 16 on the pan, produce turning moments of oppositesense in the two longitudinally spaced and parallel pivot members 26 ofthe weigh scale 10, such turning moments tending to rotate the pivotmembers about their own longitudinal axes, such axes being parallel andin a plane which is parallel to the plane of the pan 10 andperpendicular to the longitudinal axis of the overall device. Straps 28aand 28b are connected at the top to the base rail 14c and dependvertically therefrom so as to be secured at the bottom of the outsidesurface of the pivot member 26 shown in FIGS. 2, 3, and 4. Strap clamps30a and 30b are provided for clamping the strap to the base rail 14c asbest shown in FIGS. 2 and 3. Strap clamps 31 are provided at the lowerend of each of the straps 28 to secure the straps to the pivot member 26as best shown in FIGS. 3 and 4. Note in FIG. 2 that recesses 36a and 36bare provided to accommodate the clamps 30a and 30b respectively. Inaddition, straps 32a and 32b are secured at the tops thereof to theinside lateral surface of the pivot 26 and depend vertically therefromto be secured at the bottom to the outside surface of the transverselyextending load receiving rail 24. Upper strap clamps 34a and 34b areprovided for securing the strap to the pivot 26 while lower strap clamp37 is evident in FIGS. 3 and 4 to secure the straps at the lower endthereof. As is evident in FIGS. 2, 3, and 4 there are four straps ateach end of the apparatus 10 for a total of eight straps each of whichis made of spring steel and measures approximately 1.5 inches in widthby 3.5 inches in length by 0.006 inches in thickness. All of thedimensions given in this application are strictly by way of illustrationand none is to be construed as critical unless so identified.

The disposition of the straps as best shown in FIG. 3 is such as tostably support the pivot 26 between the pan and the base such that whena load is applied to the pan 12 it is transmitted directly to thetransversely extending member 24 which, as previously described, isspaced from the base 14. Therefore this load tends to pull down on thestraps 32a and 32b. Since the vertical reaction force in straps 28a and28b appears at the longitudinally opposite surfaces and in the oppositesense the applied load tends to rotate pivot 26 in the clockwisedirection as shown in FIGS. 3 and 4. It will be understood that since areversely similar apparatus is provided at the right end of the assembly10 as partially shown in FIGS. 2, 3, and 4, a reversely similar pivotexperiences a turning moment of opposite sense in response to theapplied load.

To resist as well as to measure the turning moments a solid aluminumbeam 40 is mechanically connected to and between the pivots 26. This maybe accomplished by machining suitable apertures through the sides of thepivots 26 and forming flats on the otherwise round stock of the beam 40,inserting the flat ends of the beam 40 into the apertures and providingset screws such as shown in FIGS. 3 and 4 for securing the beam 40 inplace. Since the beam, although substantially rigid, is actually subjectto bending deflection, the moment being substantially constant acrossthe length of the beam 40 for a centered load. However, the moment atprecisely the center of the beam 40 is of a given value irrespective ofthe position of the object 16 on the pan 10 and hence the point ofapplication of the load. For this purpose, strain sensing meanshereinafter described in greater detail are disposed on the beam 40 asshown in FIGS. 3 and 4 for the purpose of sensing the bending stress andgenerating an electrical signal quantity related thereto, suchelectrical signal quantity being indicative of the magnitude of theapplied load and hence being applicable to the display device 20 in themanner described with reference to FIG. 1. A pleated flexible boot 42 ispreferably clamped onto the beam 40 so as to protect the sensitivecomponents of the strain sensing apparatus.

Antisway straps 44a and 44b are preferably provided, such straps beingsecured at the centers thereof to the base 14 and at the outer endsthereof to the members 24 to prevent any longitudinal swaying of theassembly comprising the pivot which, as will be apparent by inspectionof FIGS. 3 and 4, actually floats between the pan and the base toprovide the transformation of the applied load into the turning momentsas previously described.

Continuing with the description of FIGS. 2 through 4 it can be seen thatcutouts 38a and 38b in the laterally extending member 24 provideclearance for the clamp blocks 34a and 34b respectively and similarly arecess is provided at each end of the pivot 26 as best shown in FIGS. 3and 4 to accommodate the clamp blocks 37. The member 24 is machined toprovide a relief 44 in the center thereof to accommodate the beam 40 asbest shown in FIG. 3. As also shown in FIGS. 3 and 4 cushion typespacers may be afforded between the clamp blocks and the straps asdesired. Various other details also appear from the drawings and,because they will be of apparent importance to one skilled in the art,no specific description will be provided.

To increase the sensitivity of the weigh scale device the center of thebeam 40 is flattened as shown in FIGS. 3 and 4 to provide parallel flatsurfaces 46 and 48 on which are bonded adjacent sets of strain gageresistors, two adjacent resistors on the top flat 46 and two adjacentresistors on the bottom flat 48. The resistors 50a and 50b arepreferably disposed on the top flat 46 as shown in FIG. 5a with thesecond pair of resistors 50c and 50d being disposed on the bottom flatin substantially the same orientation. In this orientation, theresistors on flat 46 respond to compression in the beam 40 while theresistors on flat 48 respond to tension. Thus, bending stress caused bythe turning moments is sensed. Although it is not essential, it ispreferred to connect the four strain gage resistors into a bridgecircuit as shown in FIG. 5, so that the tension and compression valvesadd. A dc source is connected across one pair of terminals as indicated,the other terminals serving as outputs to the display device aspreviously described. In FIG. 5 the output terminals are furtherprovided with temperature sensitive compensating resistors 52 and 54 tocompensate for temperature changes which may occur between originalcalibration of the device and the actual time of use.

The operation of the device of FIGS. 1 through 4 is believed to beclearly apparent from the somewhat mechanically schematisized drawing ofFIG. 6. In FIG. 6 the pivots 26' and 26" are represented by round barsdisposed in spaced parallel relationship to one another and being ofequal length. The two round bars representing the pivots are joined byand mechanically connected to opposite ends of the beam 40 which isconfigured such that the center of moment measurements is in a planewhich is equidistant to the axes of rotation of the pivots 26 andperpendicular to the plane which includes the two axes. The deflectionof the beam 40 due to the moments of opposite sense is indicated by thephantom line in FIG. 6 and of course is greatly exaggerated for thepurpose of illustration. In FIG. 6 the load applying member comprisingthe pan 12, the rails 22, and the lateral members 24 are missing but itis to be understood that the application of a load pulls downwardly onthe straps 32a, 32b, 58a and 58b. The fraction of the load which appearsin each of the straps will, of course, depend upon the position of theload on the pan but, as previously described, with the center of themoment measurement in the position indicated the measured moment will bethe same irrespective of the position of the load and the division ofthe load force as between the straps. The downward force which isapplied to the pivots by the straps 32 and 58 is opposed by the upwardforce of the straps 28 and 56 and hence the moments of opposite sense asillustrated in FIG. 26 are produced in the pivots 26. The concept of astrap which is actually wrapped around a pivot member having acylindrical or tubular configuration may not be necessary in actualapplication as, from the description of FIGS. 1 through 4, it isapparent that actual angular deflection of the pivots 26 is extremelyslight. However, a system of greater angular deflection may of course beconstructed using more compliant materials in which circumstances it maybe advantageous to employ straps such as those highly schematicallyshown in FIG. 6 which wrap around the pivot members to at least alimited degree thus to avoid any variation in the effective radiusbetween the center of the pivot member and the point of tangentialapplication of the applied load.

Looking now to FIGS. 7 through 9 a split scale device is shown whichpermits the same physical apparatus to weigh objects in weigh rangeswhich differ by an order of magnitude while retaining the originalsensitivity and accuracy of the scale. In FIG. 7 the weigh scaleapparatus comprises a base 14 identical to the base previously describedwith reference to FIGS. 1 through 4 and a pan 60 of generally similarconfiguration to the pan previously described but having a smallinterior area 62 upon which light and small objects may be placed toweigh according to a scale factor which is ten times the scale factoremployed for the surrounding portion of the pan 60. This convenientscale factor variation is provided as follows: pan 60 is interconnectedwith the pivots in the fashion illustrated in FIGS. 2 through 4 (in thiscase the pivots are designated by reference characters 74 and 76,respectively) to measure to a scale factor of 1; pan portion 62 isconnected through vertical rigid strut 64 and horizontal cross piece 66to the interior ends of oppositely extending lever arms 68 and 70 whichare secured to the pivots 74 and 76 as best shown in FIG. 9.Accordingly, the objects disposed on pan 62 produce forces which areapplied to the pivots through fairly long lever arms tending to multiplythe moments by a factor of ten. Multiplication of the moment obviouslymultiplies the sensitivity of the scale. The use of the forcemultiplying lever arms may be employed with or without the split panconcept shown in FIG. 6 as will be apparent to those skilled in the art.

Various other structural modifications may be employed with or embodiedin the apparatus which has been described herein; for example, the beam40 which is shown in the drawings as being of round or circular crosssection stock may readily be replaced with an I-beam having a flexurearea defined by a segment from which the flange, both top and bottom,has been removed and in which an oblong aperture may be formed for thereceipt of the strain gage resistors. Many other beam configurationswill also be apparent. The straps, although preferred, may be replacedin various devices with cables, knife edges, and other structures havinggood torsional compliance. Also, as shown in FIG. 2, a small vibratorunit 80 may be mechanically interconnected with the floating measurementsection of the weigh scale 10 to provide a dithering function; i.e., alow order constant frequency vibration of the pivot construction thus toovercome any hysteretic effects of friction in the apparatus. It mayalso be desirable to provide means for mechanically nulling out the tareweight of the pan and fasteners to reduce or relax bending stress on thebeam except when a load is applied. Although the invention has beendescribed with such specificity as to satisfy the patent statutes, it isto be understood that the descriptions are not to be construed aslimiting the inventions as various other implementations thereof will beapparent to those skilled in the art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Floating beam weighscale apparatus comprising a base, a load receiving member spaced fromthe base, first and second pivot members disposed between the base andload receiving .[.members,.]. .Iadd.member, .Iaddend.load transmittingmeans interconnecting the pivots, the base, and the load receivingmember to produce moments in the pivots of opposite sense and aboutspaced axes, a substantially rigid beam connected to and extendingbetween the pivots, and means carried by the beam for producing a signalrelated to the bending stress therein over the elastic bending rangethereof.
 2. Apparatus as defined in claim 1 wherein the transmittingmeans comprises torsionally compliant straps connected in opposing pairsto the pivots, one such strap at each end being connected between thebase and the pivot and another such strap at each end being connectedbetween the pivot and the load receiving member to produce the turningmoment of the pivot.
 3. Apparatus as defined in claim 1 furtherincluding means for dithering the pivots by mechanical vibrationthereof.
 4. Apparatus as defined in claim 1 wherein the beam comprises asolid member of circular cross section over a substantial portion of thelength thereof.
 5. Apparatus as defined in claim 4 wherein the beamfurther comprises a central area having opposing and parallel flatsurfaces formed therein substantially in the center of the beam forreceiving the means for producing the signal upon elastic deformationthereof. .[.6. Apparatus as defined in claim 1 further including leverarms connected to the pivots and extending longitudinally of the scaleand perpendicular to the pivots and means interconnecting at least aportion of the load receiving member to the lever arms for multiplyingthe turning moment applied to the pivots by the load applied to the loadreceiving means..]. .[.7. Apparatus as defined in claim 6 wherein theload receiving means comprises a pan having a first surface which isdirectly connected to the pivot members and a second portion at leastpartially bounded by the first pan portion and connected to the pivotsby way of said lever arms thereby to provide a split scale apparatus..].8. Apparatus as defined in claim 1 wherein the means carried by the beamincludes at least one strain gage resistor intimately bonded to the beamand responsive to the bending stress therein to produce a variableelectrical resistive characteristic.
 9. Apparatus as defined in claim 8wherein the beam is configured to define an area of reduced crosssection between the ends thereof thus to concentrate the bending stress,the strain gage resistor being disposed on the area of reduced crosssection.
 10. Apparatus as defined in claim 9 wherein the flexure area ofreduced cross section is substantially in the center of the beam. 11.Apparatus as defined in claim 1 wherein the base comprises asubstantially rectangular assembly of rigid material, the load receivingmember comprises a pan and at least one force transmitting rail securedto the pan and extending longitudinally between the spaced pivotmembers, a pair of laterally extending elements, one of said elements ateach end of the longitudinal rail, the force transmitting meanscomprising straps connected between the laterally extending members andthe pivots at each end of the apparatus for producing a turning momentin the pivot when force is applied to the pan.
 12. Apparatus as definedin claim 11 wherein the beam is mechanically connected to the pivotmembers at the centers thereof.
 13. Apparatus as defined in claim 12further including means connected between the base and the pan toprevent longitudinal relative movement therebetween.
 14. Apparatus asdefined in claim 1 wherein the means for producing the output signalcomprises four substantially similar strain gage resistors, two of saidresistors being disposed on one side of the beam so as to respond tocompression of the beam and the others of said resistors being disposedon the other side of the beam so as to respond to tension in the beam,said four resistors being electrically interconnected into a bridgecircuit of such configuration as produce an output signal upon bendingof said beam in such a way as to subject two of said resistors tocompression forces and the other two of said resistors to tensionforces.
 15. Apparatus as defined in claim 14 further including digitaldisplay means responsive to the signal produced by said bridge circuitto produce a visual indication of the magnitude thereof.
 16. Apparatusas defined in claim 14 further including at least two temperatureresponsive compensating resistors connected to opposite terminals ofsaid bridge circuit to compensate the bridge circuit for temperaturevariations.
 17. In a weigh scale: a load pan, a pair of spaced members,means connecting the pan to said members to produce in said membersturning moments of opposite sense and related to loads applied to thepan, a beam of rigid material connected between said spaced members andof substantially greater length than thickness, and means for detectingthe extent of bending of said beam as an indication of the magnitude ofthe load applied to the pan.
 18. Apparatus as defined in claim 17further including mechanical vibrator means for dithering the weighscale. .Iadd.19. Floating beam weigh scale apparatus comprising:a base;a load receiving member spaced from the base; first and secondelongated, torsionally rigid pivot members disposed between the base andthe load receiving member; a substantially rigid beam having its endsconnected substantially centrally of and to the pivot members so thatthe beam extends between the pivot members; load transmitting meansspaced from the beam for interconnecting the pivot members, the base andthe load receiving member to produce turning moments in said pivotmembers of opposite sense and about spaced axes, said turning momentsbeing transmitted to the ends of the beam through the lengths of thepivot members to create bending stress in the beam without applyingdirect forces to the beam; and means carried by the beam for producing asignal related to the bending stress therein over the elastic bendingrange thereof..Iaddend. .Iadd.20. Apparatus as defined in claim 19wherein the load transmitting means comprises torsionally compliantstraps connected in opposing pairs to the ends of the pivot members, onesuch strap of each pair being connected between the base and a pivotmembers, and another such strap of each pair being connected between thepivot member and the load receiving member, said pairs of strapsoperative to produce the turning moments in the pivot members. .Iaddend..Iadd.21. In a weigh scale: a load pan: a pair of spaced elongated andtorsionally rigid members; a beam of rigid material having its endsconnected substantially centrally of and between said spaced members andbeing of substantially greater length than thickness; means spaced fromthe beam connecting the pan to said members to produce turning momentstherein of opposite sense and related to loads applied to the pan, saidturning moments being transmitted to the ends of the beam through saidmembers to create bending stress in the beam without applying directforces to the beam; and means for detecting the extent of bending ofsaid beam as an indication of the load applied to the pan. .Iaddend.